1. Field of the Invention
This invention deals with heat recovery from wet wood waste or other biomass material and certain fuels such as peat. Of particular interest is wood waste generated by wood processing facilities, commonly called "hog fuel".
2. Description of the Prior Art
In the past few years as fossil fuel costs have escalated, operators processing wood as a raw material, especially in sawmills, pulp and composite wood products operations, have become more interested in recovering the heating value of wood wastes that are otherwise unsuitable for conversion into salable products. Many facilities generate a sufficient amount of such waste to meet significant portions of the energy requirements of the facility. Others have access to supplies of peat which, if a suitable means of heating value recovery was available, could constitute a low cost replacement for fuel oil or natural gas.
Wood wastes from sawmilling and related raw wood handling operations have a number of characteristics that make efficient recovery of heating values difficult. Hog fuel is generally wet, substantially in excess of 50 percent by weight moisture and often in excess of the 68% moisture limit of self-sustaining combustion. Each mill source of waste has its own characteristic moisture content. A major source of waste is hydraulically removed bark, for example. While sawmill wastes such as sander dust, sawdust and shavings are relatively drier, they are usually accumulated and stored out in the weather and thus soak up rainwater during wet periods of the year.
A second problem with hog fuel is that it is very irregular in particle size distribution. Hog fuel wastes are generated from every wood handling and processing operation. The wastes range from sander dust of 0.1-3 mm diameter particle size to bark and low yard debris which may exceed dimensions of several inches in diameter by several feet in length.
A common practice in the past has been to burn wet hog fuels "as is", on a grate in a combination oil-wood waste boiler. Supplemental oil is generally used to sustain combustion and permit the boiler to follow process demands for steam. Peat and other biomass matter are similar to hog fuel in that they are wet and of unsuitable physical form or size. Thus, these potential fuels are generally not utilized in many parts of the world. While the discussion here focuses upon wet wood waste or hog fuel, the invention is applicable to any wet organic vegetable matter.
Recent improvements in heat recovery from hog fuel require a reduction in moisture content of the hog fuel before it is fed to the boiler. Studies show that reducing the initial moisture content of the fuel improves steam production and reduces boiler stack emissions. The hog fuel burning process need no longer supply all the latent heat necessary to dry the fuel. The dry fuel requires less excess air and thus boiler heat losses are reduced, improving overall thermal efficiency. The resulting high combustion zone temperatures apparently insure incineration of particulate matter before it escapes out the stack.
A state-of-the-art process that successfully accomplishes the pre-drying and burning of hog fuel is described by Spurrell in U.S. Pat. No. 4,235,174, issued Nov. 25, 1980 and assigned to Weyerhaeuser Company. In this process a portion of the largest size material from the hog fuel pile is combusted in a fluid bed burner. The products of combustion from the fluid bed are then used to dry the balance of the hog fuel pile in a rotary dryer before it is fed into a combination oil-wood waste boiler. The dried fuel is separated by size. The coarse fraction, at about 35 percent moisture burns on a furnace grate while a fines fraction at 15 percent moisture and a particle size of less than 1/8 inch (3175 microns) diameter is injected in air suspension into the boiler.
The Spurrell process, however, requires an oil pilot on the injected fines portion of the fuel in order to sustain stable combustion. The oil pilot represents a substantial use of fossil fuel, up to 30% of the total burner rating in terms of BTUs per hour at full burner loads. This usage of expensive fossil fuel is particularly unsatisfactory since it is not needed for its energy value per se but only to serve as an ignition energy source to achieve stable burning of the hog fuel material.
Attempts to burn the dried fines stream produced in the Spurrell process in air suspension without pilot or fire on the grate have been generally unsuccessful. Trial burns of this material, which is about 100 percent minus 3175 microns in size, would not sustain stable combustion without an oil pilot. Even with the pilot, overall furnace conditions were unstable, producing large swings in boiler pressures, unless a grate fire was present.
Certain wood wastes have in the past been recognized as burnable in furnaces without oil support or grate. For example, sander dust which is of very fine particle size distribution and about 5% moisture content has been burned successfully in air suspension. Schwieger, in an article entitled "Power from Wood", Power, Vol. 124, No. 2, p 51-32 (February 1980), describes sander dust, at about 12% moisture, as being fired to a package boiler. The average size of this material is said to be about 793 microns. Even so an oil pilot is recommended, suggesting unstable combustion conditions.
Special materials such as sander dust, however, generally constitute only a very minor portion of the hog fuel pile which accumulates at the typical lumber mill, particularly those integrated with pulp production facilities. The amounts of these special dry, fine wood wastes at most facilities are not, in general, sufficient to meet a significant percentage of the energy requirements of the typical mill. At many facilities generating wood wastes, however, the hog fuel pile as a whole has this capability.
Certain larger size and higher moisture ranges of wood material can be burned without oil support in refractory lined furnaces or kilns. In a refractory furnace the firebox is lined with ceramic which attains a temperature of roughly 1500.degree. F. or higher. The hot gases then contact the steam generating tubes. The heat retained by the mass of ceramic is continually re-radiated to help sustain stable combustion in the fire box, permitting otherwise difficult to burn materials or wastes to be burned without oil support. Refractory furnaces have a high initial cost and the effects of high firebox temperatures result in high maintenance costs. Again, only a small portion of the hog fuel pile is of suitable size for such combustion.
Industry, because of lower capital costs of construction and lower maintenance costs, favors the use of "water wall" boilers wherein the flame is substantially surrounded by water tubes which generally reach only about 600.degree. F. In these boiler configurations, the walls are relatively cold compared to the flame and are heat absorbers. Thus, there is reduced radiation assistance from firebox ceramics to help sustain the ignition process. As a result, water wall boilers are incapable of sustaining suspension firing of conventionally available hog fuel size ranges without the use of a fossil fuel pilot to provide ignition energy to continually raise the air-fuel mixture to ignition. Ignition occurs when a sufficient level of volatiles is generated from the fuel and the volatiles are mixed with air and heated to ignition temperature.
The most recent approach to burning the larger fraction of the hog fuel pile has involved pulverizing the hog fuel to a smaller particle size range. However, because of its fiber content hog fuel is inherently more difficult to pulverize than coal, for example. Eneroth, et al. in U.S. Pat. No. 4,229,183 teach improved hog fuel burning by simultaneously drying the fuel to 10-15% moisture and grinding it to a finely distributed or powder state. The flow from the pulverizer enters a cyclone which separates the fuel from the air flow. The fuel is then re-suspended in air and injected into a boiler. No grate is required. Fagerlund, in "How Some Scandinavian Mills Get Higher Fuel Value From Bark", TAPPI, Vol. 63, No. 3, pp. 35-36 (March 1980) describes the Eneroth method as grinding the wood fuel down to a particle size of 1-3 millimeters (1000-3000 microns). An oil pilot equivalent to 5% of the burner rating is recommended for flame control. Fagerlund notes that control systems in the future will be developed so that no auxiliary oil will be needed.
In another hog fuel burning system, described by Baardson in U.S. Pat. No. 3,831,535, wood waste is dried and pulverized to a maximum particle size of 5/16" or 7940 microns. This material is accumulated in a bin and injected for combustion in a refractory lined chamber where radiation from the refractory provides support for stabilized combustion.